The invention is generally directed to electronic sampling circuits such as sample and hold detectors, and particularly to a gating network for activating such sampling circuits for precise, selected intervals.
Conventionally, sampling circuitry is employed to sense the value of a selected portion of a signal received by the sampling circuitry. For example, a television signal containing a color burst followed by video information may be applied to a sampling circuit to sense the amplitude of the color burst. A gating network is usually provided to turn the sampling circuit on only during the color burst interval, thereby to inhibit sampling of the rest of the television signal. To turn on the sampling circuitry at the correct time, the gating network usually receives a gate pulse which is time coincident with the sampling component (the portion of the signal to be sampled).
One problem with conventional gating networks is the way they respond to impulse noise in the gate pulse. When such noise occurs, it may cause positive-going and negative-going noise spikes to appear both in the gate pulse and in the sampling component. A gating network which activates the sampling circuitry in response to positive-going gate pulses also ordinarily activates the sampling circuitry in response to positive-going noise spikes. Hence, the sampling circuitry will sense not only the sampling component but also a positive-going noise spike. A negative-going noise spike in the gate pulse causes the gating network to disable the sampling circuit for the duration of the negative-going noise spike. A corresponding negative-going noise spike in the sampling component is, therefore, not sensed by the sampling circuitry. This causes the output of the sampling circuit to be greatly influenced by the positive-going noise spike.
Another problem occurs when the duration of the sampling component is very short and the sampling circuitry is very fast. For example, if a conventional color burst of about 2.7 microseconds duration is to be sampled, the sampling circuitry must be turned on quickly when the color burst interval begins, and turned off no later than the end of the color burst interval. Holding the sampling circuitry on for longer than the color burst interval allows fast acting sampling circuitry to change its output to correspond to the signal value which occurs after the color burst. Hence, the sampling circuitry's output will tend to miscrepresent the true value of the color burst.
For the foregoing reasons, conventional networks for gating sampling circuitry on and off tend to cause the sampling circuitry to develop unreliable outputs. This is particularly true when the signal to be sampled is of very short duration, and when impulse noise is present.